<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        //编写函数，判断一个字符串的内容是不是纯数字
        var str = prompt("请输入一个字符串");
        function num(str) {
            if (str == 0) {
                return true;
            }
            if (str == "") {
            }
            return !!Number(str);
        }
        var res = num(str);
        if (res == 0) {
            alert("不是数字");
        } else {
            alert("是数字")
        }
    </script>
</body>

</html>